Revisiting nitrate concentrations in the Des Moines River: 1945 and 1976-2001

Area of Science:

• Environmental Science
• Hydrology
• Water Quality Analysis

Background:

• Riverine nitrate (NO3) concentrations in the north-central USA generally increased in the late 20th century.
• A previous study suggested similar NO3 levels in the Des Moines River in 1945 and the 1980s, despite increased watershed nitrogen input.

Purpose of the Study:

• To re-evaluate historical and contemporary NO3 concentrations in the Des Moines River near Des Moines, Iowa.
• To address potential errors in historical data, variations in sampling methods and locations, and incorporate new data.

Main Methods:

• Comparative analysis of historical (1945) and contemporary (1976-2001, post-1990) NO3 data.
• Statistical modeling to account for differences in sample collection (composite vs. grab) and water yield.
• Evaluation of sampling site locations and historical data accuracy.

Main Results:

• An earlier study incorrectly compared flow-weighted averages to arithmetic averages.
• Differences in sampling methods and locations influenced intertemporal comparisons.
• Estimated 1945 arithmetic average NO3 was 44-57% of the 1976-2001 mean.
• Estimated 1945 flow-weighted average NO3 was 54-73% of the 1976-2001 mean.

Conclusions:

• Nitrate concentrations in the Des Moines River in 1945 were significantly lower than in the contemporary period.
• The difference in NO3 concentrations is larger than previously reported.
• Methodological inconsistencies in historical data analysis can lead to inaccurate environmental trend assessments.